This invention relates to an exhaust gas recirculation system for an internal combustion engine, particularly a diesel engine.
Hitherto, an exhaust gas recirculation (EGR) system for recirculating a portion of exhaust gases, discharged from an internal combustion engine, to the air intake system of the same engine has been utilized as one of the effective means to minimize the emission of an obnoxious NO.sub.x component contained in the exhaust gases. When it comes to a diesel engine, no throttle valve is employed in an air intake system because of the peculiar structural design and, therefore, the difference in pressure between the air intake system and the exhaust system is so small that difficulty is encountered in acquiring a necessary quantity of exhaust gases to be recirculated through the EGR system. This is a serious problem because the diesel engine requires a large quantity of exhaust gases to be recirculated in order for the emission of the NO.sub.x component of the exhaust gases to be effectively minimized in view of the fact that the diesel engine requires a relatively large amount of air supply.
Apart from the above, the use of a supercharger in the air intake system is practised to increase the engine power output. However, in such a supercharged engine, since the pressure in the air intake system is apt to be higher than that described above, there is a problem in that a necessary quantity of the exhaust gases to be recirculated can not be obtained. Although this problem appears to be solved when design is made to supply a portion of the exhaust gases into the air intake system at a position upstream of the supercharger, this design is problematic in that a blower tends to be soiled with carbon component of the exhaust gases, and therefore, it is necessary to employ such an alternative design to supply the recirculated exhaust gases into the air intake system at a position downstream of the supercharger, with the problem consequently far from being solved.
Against the first mentioned problem, it has been suggested to install a throttle valve in the air intake system for reducing the rate of flow of air being sucked, thereby to reduce the pressure inside the air intake system when the throttle valve is closed during the recirculation of the exhaust gases, wherefor the difference in pressure between the air intake system and the exhaust system is increased to secure the necessary quantity of the exhaust gases to be recirculated.
Against the second mentioned problem, it can be contemplated to interrupt the supply of the supercharged air during the recirculation of the exhaust gases to allow the pressure inside the air intake system to decrease in order to secure the necessary quantity of the exhaust gases to be recirculated. Where the mere interruption of the supercharged air supply would not ensure the necessary quantity of the exhaust gases to be recirculated, the use of a throttle valve in the air intake system can be contemplated to allow the throttle valve to be closed a predetermined degree simultaneously with the interruption of the supercharged air supply so that the pressure inside the air intake system can be further reduced in order to secure the necessary quantity of the exhaust gases to be recirculated.
Specifically, in the design wherein the pressure inside the air intake system is reduced during the recirculation of the exhaust gases in order to secure the necessary quantity of the exhaust gases to be recirculated, the necessary quantity of the exhaust gases to be recirculated can be obtained by controlling the pressure inside the fuel intake system. However, where the exhaust gas recirculation is to be interrupted, for example, curing the exhaust gas recirculation being effected, that is, while an EGR control valve in the EGR system is opened on the one hand and the throttle valve in the air intake system is closed the predetermined degree on the other hand, the closure of the EGR control valve before the complete opening of the throttle valve creates an unexpected negative pressure inside the air intake system downstream of the throttle valve, resulting in that the temperature of the air being compressed in the engine cylinder will not increase sufficiently and the failure to ignite fuel oil sprayed into the engine cylinder may occur with the consequence of emission of exhaust gases containing a relatively large amount of unburned compound such as HC. Even if the failure to ignite the fuel oil does not occur and the latter is barely ignited, the temperature of the air being compressed may be so low as to result in the retarded ignition with the consequently increased possibility of occurrence of engine knocking. Accordingly, where the EGR system is to be employed in association with the diesel engine, countermeasures have to be taken to avoid the emission of a relatively large amount of the unburned exhaust gas components and also the occurrence of the engine knocking which would be apt to occur about the time the exhaust gas recirculation is to be interrupted.
On the other hand, in the diesel engine, although the emission of the NO.sub.x component contained in the exhaust gases can be reduced when the exhaust gas recirculation is effected, the emission of smoke tends to be inversely increased. Accordingly, these incompatible problems have to be resolved simultaneously in order to provide a clean diesel engine. In this respect, experiments have shown that the emission of smoke can be minimized when the compression ratio is increased by way of, for example, increasing the supercharged pressure, and, therefore, the minimization of both the emission of the NO.sub.x component and the emission of the smoke can be accomplished if the EGR system and the supercharger are coordinately controlled.
Moreover, considering the fact that, when the exhaust gas recirculation is abruptly switched on and off alternately, an abrupt change in torque will occur with the result that the drivability will be reduced. In view of this, it is advisable for the change-over of the exhaust gas recirculation to be carried out in such a way as to gradually increase or decrease the rate of flow of the exhaust gases being recirculated.